Dual counter-rotating crankshafts may be employed in an engine in order to cancel torque pulsations. For example, an engine including dual counter-rotating crankshafts may have a rigid coupling between the crankshafts, so that the acceleration of one crankshaft is counteracted by acceleration of the other crankshaft in the opposite rotational direction. Accordingly, reaction torque imposed upon a powertrain support structure (e.g., engine mounts) may be reduced, and correspondingly noise and vibration sensed inside a vehicle's passenger compartment may be reduced.
One example of a dual counter-rotating crankshaft engine is provided by Berger et al. in U.S. Pat. No. 7,533,639. In this example, vertically oriented cylinders coupled to each crankshaft are paired and aligned in parallel. The dual counter-rotating crankshafts mutually cancel each other's vibrational torque reaction against the powertrain support structure, resulting in a reduced vibration level that lowers the “lug limit,” or the feasible engine speed where a torque converter coupled to the engine can remain locked before noise, vibration, harshness (NVH) levels cause the torque converter to be unlocked. The lower lug limit allows for greater efficiency and fuel economy because the torque converter may remain locked at lower engine speeds relative to a torque converter coupled to a single crankshaft engine. The reduced vibration of the powertrain may also enable more extensive lugging operation with some of the engine's cylinders not firing.
However, the inventors herein have identified potential issues with such an approach. As an example, since the vertically oriented cylinders of a four cylinder engine are paired and arranged in parallel, inertial forces acting on the reciprocating pistons cause an inherent unbalanced vibration that requires balance shafts to counteract the vibration in order for the engine to be mechanically balanced. The addition of balance shafts to the engine increases cost, weight, inertia, and friction of the engine. Moreover, the vertical orientation of the cylinders causes the height of the engine to be taller, which may be undesirable for engine fitting, vehicle styling, and raises the center of gravity of the engine, which may affect stability and control of a vehicle in which the engine is installed.
At least some of the above issues may be addressed by an engine including a first crankshaft, a second crankshaft coupled with the first crankshaft such that the first crankshaft and the second crankshaft are horizontally coplanar, a first piston operable to reciprocate in a first horizontal cylinder via coupling with the first crankshaft, and a second piston operable to reciprocate in a second horizontal cylinder that is opposing and horizontally collinear with the first cylinder via coupling with the second crankshaft.
By orienting the cylinders on each of the dual counter-rotating crankshafts so that they are opposing and horizontally collinear, the pistons can be reciprocated in a balanced manner without the need for balance shafts to maintain mechanical balance of the engine. In this way, the dual counter-rotating crankshaft engine may be mechanically balanced without the cost, weight, inertia, and friction penalties attributed to balance shafts. Moreover, by orienting the cylinders horizontally, the center of gravity of the engine may be lowered to provide greater stability relative to an engine with vertically oriented cylinders.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.